Aluminium is produced by the electrolysis of alumina dissolved in a cryolite-based molten electrolyte. The electrolytic cell known as the Hall-He,acu/e/ roult cell, uses anodes consisting of a compacted mixture of petroleum coke and pitch.
Although significant technological refinements in the cell design and construction have been made, the basic process of producing aluminium still remains substantially similar to what it was one hundred years ago.
The carbon anode blocks are consumed during electrolysis and must be replaced every four or five weeks of operation in standard cells. The oxygen resulting from the decomposition of alumina burns the carbon anode at a theoretical rate of approximately 330 kg per ton of aluminium produced, but in practice the carbon consumption is about 450 kg per ton of aluminium due to side reactions. This results in the emission of carbon oxides, sulfur oxides and other undesirable gases which are now being recognized as major atmosphere pollutants, but such emissions are considerably less dangerous and less polluting than those produced during fabrication of the carbon anodes with pitch as a binder.
At present, the method of producing carbon blocks to be used as anodes in aluminium production cells consists of mixing petroleum coke with pitch followed by compacting and calcining. Calcining designates a baking process in which volatiles are driven off at high temperatures without fusing the material.
The fabrication of the carbon anodes involves the use of pitch. During fabrication, gases are emitted especially from the pitch during the long period required for calcining the blocks. These gases are polluting and very dangerous to the environment and are recognized as a major hazard to the health of workers involved in the production.
The pitch serves as the binder for the dry mixture of carbonaceous materials. Unfortunately, the pitch binder presents a series of serious hazards for health and for the environment.
Both solid and liquid pitch is used. The utilization of solid pitch results in unsatisfactory working conditions for the workmen, such as irritation and itching of the skin and eyes, and special precautions must be taken in order to protect the workmen during all operations where pitch is involved.
Additional problems result from the utilization of liquid pitch, particularly in regard to the storage and transportation to the utilization plant.
During the calcining of the carbon blocks, which is required to eliminate the volatile components and stabilize the blocks, there is an emission of aromatic polycyclic hydrocarbons (PAH), which are very dangerous to the health and special equipment is required to absorb these products. However, the residual products after absorption are also difficult to eliminate and the cost of disposal is high.
Utilization of pitch as a binder requires the mixing operations with carbonaceous materials to be carried out at about 150.degree.-200.degree. C. and this creates operating complications and high operating costs.
The calcining process is complicated and costly and large furnaces are required which are difficult to operate, are polluting and expensive because of the high energy consumption. On account of the requirement of low thermal gradients during heating of these blocks to over 1000.degree. C. and later cooling, the calcining operation normally takes as long as 2 to 4 weeks.
An additional disadvantage of the pitch is due to the fact that when the blocks are calcined the pitch is transformed mainly into a form of carbon which oxidizes more rapidly than the petroleum coke. This leads to disintegration of the anode block with formation of unutilized carbon powder which is detrimental to the operation of the electrolytic cell and increases the carbon consumption.
It would, therefore, be extremely advantageous for the aluminium industry to produce carbon blocks fabricated with a non-polluting binder without the necessity of handling pitch or like dangerous materials, avoiding the emission of polluting substances. Additionally, there is a need to develop compositions and methods which eliminate the high temperature fabrication and the long calcining times required to form the carbon blocks following the current state of the art.
U.S. Pat. No. 4,613,375 describes a carbon paste for self-calcining electrodes based on 70-90 wt % hard bitumen and 10-30 wt % soft bitumen to which was added up to 2 wt % of additives including aluminium and alumina. Though bitumen contains less polycyclic aromatic hydrocarbons than pitch, these bitumen-based materials have not found acceptance.
U.S. Pat. No. 4,919,771 discloses a Soderberg anode paste or a bipolar electrode made of a mixture of carbon and alumina containing about 85 wt % alumina and 15 wt % of pitch. Because of the high electrical resistivity of this material, it was proposed to use auxiliary electrodes of synthetic graphite, but these proposals have not been proven in practice.
U.S. Pat. No. 5,110,427 describes making use of scrap plastic coated aluminium foil by grinding it and including it in a carbon anode used for aluminium production by adding the ground scrap to the usual carbon-containing masses. This, however, still requires the usual pitch binders and addition of the scrap increased the resistivity of the anode.
Co-pending application Ser. No. 07/897,726 describes a carbon containing paste for use in particular as components of electrolytic cells as such or compacted to form anodes, cathodes and cell linings of cells for the electrolysis of alumina for the production of aluminium, which consists of a compact mixture of one or more particulate carbonaceous material(s) with a non-carbonaceous non-polluting binder and optionally with one or more fillers. The binder is a suspension of one or more colloids or colloid precursors or colloid reagents optionally with one or more chelating agents. Cell components are produced by forming the paste to a desired shape and size, for instance in a mold, an injection die or in a cell bottom, compacting and drying before use, or the paste can be used as such like in a Soderberg type anode, or for bonding together carbon blocks like a ramming paste. Among various possible additives are alumina and aluminium. This approach is promising especially as regards pollution reduction in the manufacturing process, but improvements are still desirable in materials obtained.